1. Field of the Invention
The present invention relates to a liquid crystal display device and a method of producing the same. In particular, the present invention relates to a liquid crystal display device having liquid-crystal molecules which are axially symmetrically aligned within liquid crystal regions separated by a polymer wall, and a method of producing the same.
2. Description of the Related Art
Conventionally, TN (twisted nematic)- or STN (super-twisted nematic)-type liquid crystal display devices have been used as display devices utilizing electro-optic effects. A technique of increasing a viewing angle of such liquid crystal display devices has been actively developed.
As one of the conventionally proposed techniques of increasing a viewing angle of the TN-type liquid crystal display devices, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a liquid crystal display device having axially symmetrically aligned liquid crystal molecules in liquid crystal regions separated by a polymer wall,i.e., a so-called ASM (Axially Symmetrically aligned Microcell)-mode liquid crystal display device. The liquid crystal region substantially surrounded by the polymer wall are typically formed on a pixel-by-pixel basis. The ASM-mode liquid crystal display device has axially symmetrically aligned liquid crystal molecules. Therefore, the variation in contrast is small regardless of the direction in which a viewer views the liquid crystal display device. In other words, the ASM-mode liquid crystal display device has wide viewing-angle characteristics.
The ASM-mode liquid crystal display device as disclosed in the above-mentioned Japanese Laid-Open Publications is produced by polymerization-induced phase-separation of a mixture of a polymeric material and a liquid crystal material.
Hereinafter, a method for producing the conventional ASM-mode liquid crystal display device is described with reference to FIG. 9. First, a glass substrate 308 having a color filter and an electrode on one of the surfaces thereof is prepared (Step (a)). It should be noted that, for simplicity, the electrode and color filter formed on the top surface of the glass substrate 308 are not shown in FIG. 9. A method for forming the color filter will be described below.
Thereafter, a polymer wall 317 for axially symmetrically aligning the liquid crystal molecules is formed on the surface of the glass substrate 308 on which the electrode and color filter are formed (Step (b)). The polymer wall 317 is formed so as to have, for example, a grid pattern. After a photo-sensitive color resin material is applied on the glass substrate 308 by a spin coating method, the resultant glass substrate 308 is exposed to the light through a photomask having a prescribed pattern. Then, the glass substrate 308 is developed, whereby the polymer wall having the grid pattern is formed. The photo-sensitive color resin material may be either of a positive-type or negative-type. Alternatively, the polymer wall 317 may be formed from a non-photo-sensitive resin material, although an additional step of forming a resist film is required.
A pillar 320 is patterned in a discrete manner on the top of a portion of the polymer wall 317 (Step (c)). The pillar 320 is also formed by the exposure and development of a photo-sensitive color resin material.
The surface of the glass substrate 308 on which the polymer wall 317 and the pillar 320 are formed is coated with a vertical-alignment layer 321 such as a polyimide (Step (d)). On the other hand, a counter glass substrate 302 having an electrode formed thereon is supplied (Step (e)), and the counter glass substrate 302 is also coated with the vertical-alignment layer 321 (Step (f)).
These two glass substrates 308 and 302 are laminated to each other with the respective electrodes facing each other, thereby forming a liquid crystal cell (Step (g)). The distance between the two glass substrates (i.e., a cell gap; the thickness of a liquid crystal layer) is defined by the sum of the respective heights of the polymer wall 317 and the pillar 320.
A liquid crystal material 316 is introduced into the liquid-crystal cell gap by, for example, a vacuum injection method (Step (h)). Finally, by, for example, applying a voltage between the electrodes facing each other, liquid crystal molecules within a corresponding liquid crystal region 315 are axially symmetrically aligned (Step (i)). More specifically, the liquid crystal molecules within a corresponding one of the liquid crystal regions separated by the polymer wall 317 are axially symmetrically aligned with respect to the axis (which is perpendicular to the both glass substrates) as shown by a broken line in FIG. 9.
FIG. 10 shows a cross-sectional structure of a conventional color filter. A black matrix (BM) for shielding the gaps between color patterns from light, as well as color resin layers of red (R), green (G) and blue (B) corresponding to the respective pixels are formed on a glass substrate. An overcoat (OC) layer having a thickness of about 0.5 .mu.m to about 2.0 .mu.m is formed thereon in order to improve the flatness of the substrate or the like. The overcoat layer is formed from an acrylic resin, an epoxy resin or the like. Moreover, an indium tin oxide (ITO) film for a transparent signal electrode is formed thereon. The BM film is generally formed from a metal chromium film having a thickness of about 100 nm to about 150 nm. The color resin layers are formed from a resin material colored by a dye or pigment, and generally have a thickness of about 1 .mu.m to about 3 .mu.m.
The color filter is formed by a method for patterning the photo-sensitive color resin layer formed on the substrate by a photolithography technique. For example, by a series of the steps of forming, exposing and developing a photo-sensitive resin are conducted for each of the R, G and B photo-sensitive color resin materials (i.e., by conducting the series of the steps three times in total), the color filter of R, G and B can be formed. A method for forming a photo-sensitive color resin layer includes a method for applying a liquid, photo-sensitive color resin material (diluted with a solvent) on a substrate by a spin-coating method, and a method for transferring a photo-sensitive color resin material in the form of a dry film onto the substrate. By producing the above-mentioned ASM-mode liquid crystal display device by using such a color filter, a color liquid crystal display device having wider viewing-angle characteristics can be realized.
However, the above-mentioned ASM-mode liquid crystal display device and method for producing the same have the following problems: in the ASM-mode liquid crystal display device, the steps of forming the polymer wall and pillar, which are not required for the TN-type liquid crystal display device, is required in addition to the step of forming a color filter. Therefore, the number of production steps is increased, causing an increase in the cost as well as reduction in the production yield. Moreover, since the production steps are complicated, the polymer wall and pillar may not have a uniform height due to the variation in the conditions of the production steps, thereby causing a divergence in the display characteristics of the liquid crystal display device.